Currently, wrist prostheses replace both radial and carpal components of the wrist joint. In such prostheses, the amount of carpal resection is quite considerable and the resultant changes in the wrist biomechanics lead to some dissatisfaction which results in limited application and delay in the timing of the operation until the joint is in a highly advanced degenerative state.
Currently, the wrist biomechanic is not very well understood, especially the kinematics of the skafoid and the lunatum which present semifree kind of movements relative to each other. However, in current practice the carpal component is either fixed to both of these bones or, applied after resection of these bones. Beyond that, in tumor cases, affecting the articular surface located in the distal radius, when trauma or a kind of rheumatoid disease spares the carpal bones, but seriously affects the articular surface of the radius, until today there has been no joint replacement system which can reconstruct only the damaged radial part of the wrist.
The current status of prosthetic applications in hand surgery can be reviewed from the last publication of operative Hand Surgery of David P. Green-(ISBN 0-443-08803-0 Churchill Livingstone Publications pages 143-187). In addition, the Journal of American Hand Surgery could be investigated.
It is obvious that all kinds of current prostheses force the surgeon inevitably to change the whole of the joint. In case of the normal carpal side cartilage, to be obligated to apply a prosthesis to the carpal side of the joint increases the risk of complication and may be regarded as "excessive treatment".
In applications relating to the total wrist prosthesis, problems with loosening seen particularly in the carpal component and the stability lost due to the wide carpal resection are still unsolved. Therefore, many surgeons keep away from total wrist prosthesis surgery and prefer artrodesis or delay surgery until extreme deformity occurs. The new concept in accordance with this invention enables the prosthetic replacement of only the damaged part of the joint (taumatic, tumoral, degenerative and so on). Likewise with the assistance of the limited replacement arthroplastie, the original biomechanic of the wrist joint is maintained and provides a sort of profilactic surgery with conservation of wrist ligaments and undamaged parts of the joint. In daily practice, the phenemenons which disturb the morphologic and functional wholeness of such as intraarticular fractures, such as giant cell tumors generally occurs within the distal radius. In such cases, arthroplastie which reconstructs only the radial part of the joint is not described. Certainly, the prosthetic material in question will be in different types according to the location and the stage of the pathology. For instance, in case of benign tumors, a design possessing a massive stem (body) to be applied with bone cement (Methilmetakrilat) is preferable. On the other hand, in a case of intraarticular fractures, the application of cementless, low profile design will be chosen to leave sufficient bone stock.
With the new concept which we have introduced here, the realization of further experimental and clinical studies and prototype designs undoubtedly will be developed. Similarly, the fact that the distal radio-ulnar is affected or not will modify the choice of required design.
Following the arthroscopic MRI and operative findings, if only the carpal bones' articular surfaces are found degenerated, the surgeon may be content with only the replacement of the degenerated surfaces. Moreover, if only a part of radial articular surface is found degenerated (Skafoid or Ulnar fossa), again the surgeon may choose limited replacement of only degenerated parts in the form of a hemi-partial prosthetic application, sparing the rest of the joint. The insertion of the radial component may either be realized by a trans- radial -stiloid approach which is not published yet, in case of distal radiovulnar joint degeneration, the Taleisnik modification approach for Kapanji procedure (resection of distal ulnar), or finally the current dorsal approach used regularly in wrist replacement arthroplastie being for hemi-partial application.
Whether TFCC (Triangular fibrocartilage complex) is intact or not, and the carpal instability added or not, may imply little modifications in the design.
In accordance with the abovementioned principles, partial surface replacement prosthetic applications are not described and realized for finger joints. Till today surgeons exhorted arthrothesis in case of introarticular fractures or benign tumors located adjacent to finger joints. Thus the joint is sacrificed, even if one of the joint surfaces is completely intact. Furthermore, only the fracture which holds one articular surface of the MP or PIP joint may imply a contraindication for revascularisation or replantation of the finger. Therefore, a simple partial surface replacement can change the outlook of the whole finger. Also in corpometacorpal an intercarpal joints, only the damaged surfaces' anatomic replacement can be used.
The surfaces of the prosthesis may be metal or plastic in nature. The surfaces be covered with plastic or other material or, as an alternative measure some chemicals may be injected in the joint space between the prosthetic material and the opposing normal articular surface.
The prosthesis can be secured by cement or not. Bone graft, porouscoated, hidroksipatit impregrinated surface, bone-ongrowth, and bone-ingrowth phenomenons may be taken into consideration when designing the prosthesis. The components of the modular prosthesis may be fabricated to facilitate assembly of the device outside of the body, or inside the body with a different of surgical approach. Arthroscopic assistance may probably be required. With the plate and the screw application, additional primary stability can be obtained. Variations of plate and screw applications can be fixed or assembled over the material.
To increase stability, the stem and the body components may be manufactured as "expansion type". In the beginning the prototype model will possess an extension screw system, but this may be modified when necessary. Distal radial articular surface inclination angle is given generally in two different plans as 23 and 11 degree. However, we have seen considerable modifications in our measurements. Ideally we anticipate matching the angle of the articular surface with the other wrist of the patient. The modular and compact system in accordance with this invention is designed to allow this accomodation. It is obvious that this prosthetic system will be applied with the help of an instrument set to provide to find the correct size, position, and the fixation of the implant. To facilitate the insertion of the prosthesis some distraction will probably be needed. Thus a kind of external fixation device will be utilized, and in addition, multiple raspes, osteotoms and specially designed insertion devices are indispensable. These instruments will be developed in experimental studies and in following days newly designed versions will be invented. Where necessary, the prosthesis can be custom made as well.
Variations allows the fixation of the prosthesis to the bone with the help of several devices.
In the stem and or the body of the prosthesis, a rectangular area to be filled with bone graft taken with a special instrument may be helpful to contribute to the stability of the device.
In the above mentioned radial styloid approach, the main concern is to protect volar and dorsal ligaments, and thus the stability of the joint. This is a new approach not described elsewhere.
Scaphoid and lunate bones articular surfaces may be reconstructed with the help of synthetic materials initially or later. In the case of the scapho-lunate rupture which is difficult to repair, a special design to address this area may be utilized.
As it is seen, with the help of the new conceptual approach to the prosthetic surgery in the hand, it will be possible to reconstruct only the limited damaged part of the wrist joint with an anatomically designed prosthetic device. Healthy parts will be left intact, and only in the case of necessity will other parts of the joint be changed to convert to total wrist prosthesis. While doing this, and to the extent possible, any of the previously applicated prosthetic components will not be removed and may be expressed through the help of an analogy to modular furniture. In following designs, main details have been mentioned on the radial prosthetic component shown in approximately natural dimensions. It must be kept in mind that necessary modifications will be realized in the details.